The wide range of optical properties that are exhibited by materials provides a useful source of information about the identity of the materials. Different materials reflect, scatter, absorb, and transmit light in different ways, allowing the materials to be distinguished by measuring these optical properties. In particular, a lot of materials tend to reflect most of the light that is incident on their surfaces and that is not absorbed. Therefore, surface reflectance properties are important optical attributes of a material.
Acquiring realistic reflectance properties of real-world objects, such as their spatially varying colors, specularities, roughnesses, translucencies, and diffuse reflectances, can be quite difficult, however. The image acquired from an object generally depends not only on its characteristic reflectance properties, but also on the conditions of illumination. The process of capturing the radiance of light reflected by an object from a full range of incident illumination directions can be lengthy, and can require specialized equipment. Surface reflectance properties may often be crafted laboriously by hand, using image-editing software.
To address these issues, mathematical reflectance models have been developed which parameterize an object's bidirectional reflectance distribution function (BDRF). The BDRF provides the ratio of the reflected radiance to the incident flux, per unit area, for all directions of view and incident illumination. Typically, these analytic reflectance models parameterize the BDRF in terms of the diffuse color of the object, the specular color of the object, and the specular roughness parameter of the object. This parameterization allows these models to be fitted to sparser samples of the BDRF. It can be difficult, however, to reliably estimate both the diffuse and specular components of an object's surface reflectance, using these mathematical models with a limited number of samples. In particular, parameters obtained by fitting to a limited number of samples can grossly mischaracterize the specular properties of a material.
Accordingly, there is a need for an apparatus and method that allow both the diffuse and the specular reflectance properties of an object to be estimated reliably, without requiring excessive amounts of data and/or complex equipment.